30 Mar
1999
30 Mar
'99
4:42 p.m.
-----Original Message-----
From: Tony Duell <ard(a)p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
<classiccmp(a)u.washington.edu>
Date: Tuesday, March 30, 1999 4:33 PM
Subject: Re: Computer busses....
While it's true that's all you'd need, it's not all you might want, and
while I agree that you can and probably should do that, I've actually seen
it done more by using the processor to do much of the work by jamming a jump
to a front-panel-or diagnostic-card-resident monitor. I doubt, however,
that I've seen this stuff more than a dozen times altogether. I've seen
plenty of front-panels which were connected only to make the lights blink.
>
> You've hit most of the important signals. One I'd add, however, is a
data
> bus disable, and perhaps an address bus disable as
well. This would
allow a
front panel or
other bus mastering device to steal cycles under certain
circumstances.
He's got BUSreq/BUSack. That's all you really need for a DMA device (at
least if those signals have similar definitions to the Z80 ones of that
name). When the Z80 gets a Busreq, it tri-states the internal
address/data buffers IIRC, and you can use BUSack to tri-state the
external ones that you should have added. A frontpanel can easily be implemented as a DMA device
using those signals.
-tony